Devices that capture images are increasingly becoming more technologically advanced. Examples of such devices include traditional cameras, digital cameras, and video recorders. With the advancement of digital technology in particular, such devices are being used to capture images in digitized formats. The use of digital images is becoming more prevalent and diverse. For example, digital images can be stored on computer memory, transmitted over a network, and used on web-pages.
Image capturing devices utilize one or more lenses to focus an image of an object on a medium that captures the image. Image characteristics such as blurriness, brightness and intensity may be determined by factors that include the properties of the lenses, the distance between the lens and the object, and the distance between the lens and the medium.
Blurriness is an image characteristic that detracts from the clarity of a captured image. A blurred image typically appears to be out of focus to a viewer. Blurriness may result when a lens is too close or too far from a target object.
Digital imaging devices capture images on discrete light-detecting sensors, which are sometimes referred to as pixels. For an in-focus image, the lens receives light from one portion of the object to a designated pixel. In a typical digital imaging device, the image of an object is obtained in two dimensions, because imaging of the device does not carry depth information. As a result, it is difficult to correct the out-of-focus blurring.
In the past, several techniques have been proposed to remove blurriness from captured images, but past techniques have several disadvantages. Some devices, such as auto-focus cameras, reduce blurriness of captured images by adjusting the properties of the lens before the image is captured. For example, cameras may move one lens inward or outward in order to reduce the blurriness of the captured image. In such devices, the resulting image of the target object may be in focus, but the surrounding environment may be out of focus because the lens is only adjusted for the object.
Some auto-focus devices are equipped to position the lens based on a measured distance to a target object. This distance may be determined by reflecting a light of an object and using a triangulation technique to determine the distance of the object from the lens. The triangulation technique measures the distance between a limited, discrete number of points on the object and the lens. For example, some devices use only one laser pulse or beam to approximate the distance of entire target object from the lens. Other devices use four corner positions and one center position over the entire scene to determine what distance the target object is from the lens.
Technology has advanced to where imaging devices may be equipped with three-dimensional sensors that accurately measure the distance between discrete regions on the target object and the lens. For digital imaging devices, a three-dimensional sensor can measure the distance between each pixel and a portion of a target object that will reflect light onto that pixel. One type of three-dimensional sensor is described in provisional U.S. Patent application, entitled “Methods for enhancing performance and data acquired from three-dimensional image systems”, having Ser. No. 60/157,659, and a filing date of 5 Oct. 1999. This application is incorporated by reference herein in its entirety for all purposes. A device such as described in the Ser. No. 60/157,659 application has the capability of obtaining both intensity and time-of-flight information of a scene it is imaging. The scene may include several objects that are positioned different distances from the imaging device and its pixels.
A basic illustration of an imaging device 800 incorporating a three-dimensional sensor is provided in FIG. 8. The basic components of imaging device 800 include a lens 820, a field-stop (aperture stop) 830 and a two-dimensional imaging medium 840 comprising a plurality of pixels 842. Each pixel 842 uses reflected light to image a region of the target object. In addition, the pixels 842 are adapted to act as a three-dimensional sensor in cooperation with another light source reflecting off the target object. Specifically, each pixel 842 is adapted to use time-of-flight information from the other light source for the purpose of identifying a distance between that pixel and a location on the target object that is being imaged by that pixel. As shown by FIG. 8, a laser source 852 may be coupled or integrated into the device to provide the light source in which pixels 842 detect time-of-flight information. The laser source 852 emits a pulse of laser light 812. A target object 815 reflects light from multiple regions thru the lens 820. The light reflected from the target object 815 is used to create an image in the imaging medium 840. Each pixel 842 images a different portion of the target 815. Furthermore, each pixel 842 of the imaging medium 840 is able to measure both the intensity of the light received and the total time of the light travel from the laser source 852 to the imaging medium. The result is that, for each pulse of the laser, a three-dimensional view of the visible portion of the target object may be obtained.
FIG. 9 illustrates a basic imaging system comprising a lens 910 and an image capturing medium 940. An object 930 maybe located a distance s away from the lens 910. The image capturing medium 940 maybe located at a distance d from the lens 910. The lens 910 has a height a and a focal length f. The “thin-lens” equation may be used to describe the relation among s, d, and f, as follows:1/s+1/d=1/f  (1)If some point of the object is closer or farther away from lens 910 than the ideal distance s, then the lens 910 will image this portion as a circle with radius c. This circle is sometimes referred to as a Circle of Confusion (CoC).
The largest diameter of an object portion that can be imaged by the three-dimensional sensing system to a particular pixel, without any detectable blurring, is equal to a pixel length x. Therefore, when a target object is in-focus for lens 910, then the portion of the image appearing on pixels of the imaging medium is equal to x. When the target object is out-of-focus, then a portion of an image appearing on a pixel is larger than x. The image portion will then fall on more than one pixel, causing the image of the target object to appear blurred.
Another technique for removing blurring from an image is done using a field stop. The field stop narrows the gap that light can pass, thereby increasing the depth of focus. The use of field stops results in less light passing thru the lens, so that the resulting image is darker or even partly invisible.